Workflow Question — Multiple Resolutions, ideal sequence settings?

Hi there,
Sorry for what I'm sure is a stupid question that has been asked before, but there are is A LOT of conflicting information out there and I wanted to ask the experts. I'm new to Premiere Pro CC 2015 (from FCP7), am shooting a project with footage from multiple cameras and multiple resolutions — 4K, 1080p and some 'archive' material from old iPhones and other lower resolutions . . . I'm just trying to understand what is happening under the hood of Premiere Pro. The delivery is most likely going to be in 1080p but I want to leave open the possibility of something higher for the future. Should I be editing in a 4K timeline and scaling the lower resolutions up to match it, and exporting whatever my deliverable is at the end? Or start working in a 1080p timeline to begin with? What happens for instance to the native 1080p that was "scaled up" in the 4K timeline then exported back to deliverable at 1080p? Is premiere pro doing fancy math at the end export to understand all of that, or is that 1080p footage being resampled/scaled twice? Or if I zoom in (or stabilize) a 4K shot in the 4K timeline losing some sharpness in the process, will that become irrelevant once exported to 1080p? What workflow has worked best for you guys? THANKS for any info!

I appreciate it! To answer your question, the majority of footage (I'd say 80%) is being shot at 4K , while the rest might be a hodgepodge of 720p, 1080p and still photos. What's trickier is some of the lower res material I would probably want to reframe which means zooming in and recropping (beyond the upscale to 4K).
In these situations is it preferable to work at the deliverable resolution instead?

If delivering as 1080p, and using a bunch of 1080p and 720p source footage, then you do not want to edit at 4K - Premiere will "blow up" the lesser footage to 4K to match sequence setting, then when you export to 1080p, the new "4K" footage is scaled back down again, but now it is soft or pixelated.

720p footage can look good in a 1080p sequence (if not zooming in any), and 1080p footage in a 1080p sequence can have some moderate scaling applied without apparent degradation.